1. Field of the Invention
The invention relates to a process for the preparation of heat-curable silicone resins especially usable as electrically insulating resins.
2. Description of the Prior Art
It is known to produce heat curable silicone resins by the reaction of low molecular weight, multihydric alcohols at temperatures of 80.degree. to 200.degree. C. with alkoxy siloxanes having the formula ##STR2## in which OR.sup.1 represents an alkoxy group of primary or secondary aliphatic alcohols with 1 to 4 carbon atoms,
R.sup.2 represents an alkyl and/or a phenyl group, PA0 a has a value of 1.0 to 1.5, and PA0 b has a value of 0.1 to 1.0. PA0 R.sup.2 represents an alkyl and/or a phenyl group, PA0 a has a value of 1.0 and 1.5, and PA0 b has a value of 0.1 to 1.0.
If necessary, the reaction can be carried out in the presence of known transesterification catalysts and solvents. The alcohol R.sup.1 OH formed is removed and the alkoxy siloxanes are reacted with the alcohols in such relative amounts, that one SiOR.sup.1 group corresponds approximately to one COH group. The reaction is terminated at a degree of conversion of 25 to 80% by cooling to a temperature of less than 80.degree. C.
The process described in German Offenlegungsschrift No 28 28 990 wherein the preferred siloxane is is one in which a has a value of 1.2 to 1.4 and b is 0.3 to 0.8. As the OR.sup.1 group, methoxy is preferred. OR.sup.1 can also represent an ethoxy, propoxy or butoxy group and the propoxy and/or butoxy groups may be branched. In the average alkoxy siloxane molecule, a mixture of different OR.sup.1 radicals may be used with the molar ratio of methoxy or alkoxy radicals of longer chain length preferably being 2.5 to 1:1.
R.sup.2 is especially a methyl and/or a phenyl group. If methyl and phenyl groups are present simultaneously, a molar ratio of 0.4 to 1.5:1 is preferred.
The siloxane should preferably have 10 to 30 mole percent of dimethylsiloxane units and 5 to 10 mole percent of trimethylsiloxane units. Preferably, the siloxane contains 50 to 80 mole percent of monophenylsiloxane units.
As the transesterification catalysts, titanium esters or cobalt salts of organic acids or sulfonic acids, such as, p-toluenesulfonic acid or benzenesulfonic acid are preferably used.
These known silicone resins have very good thermal stability and elasticity, as well as good adhesion to a metallic substrate. A particular advantage of the cured resins is their minimal loss in weight when subjected to heat. The silicone resins described are therefore particularly suitable for use in the electrical industry.